1. Field of the Invention
The present invention relates to an excimer laser system which is used mainly as a light source of a demagnification projection aligner (which will be also referred to as the stepper, hereinafter) or a light source for processing materials and which oscillates laser through discharging excitation and more particularly, to an apparatus and method for replenishing a halogen gas into a laser chamber.
2. Description of the Related Art
When a discharging excited excimer laser system is operated with use of a laser gas containing a halogen gas, as the operating time of the laser system elapses, the material of discharging electrodes is evaporated and a chemical reaction takes place with the materials of other constituent parts, whereby the halogen gas is consumed. The consumption of the halogen gas results in that the output of the laser system is reduced.
In order to compensate for such a reduction in the laser output caused by the consumption of the halogen gas, replenishment of the halogen gas has conventionally been carried out in the following manner.
More in detail, in the case of a prior art excimer laser system, in order to excite the laser, electric energy accumulated in a capacitor of a charging circuit is provided to a discharging space within a laser chamber to activate a laser medium gas for its output. In this case, the output of the laser is increased by increasing the charging voltage of the capacitor. Accordingly, the laser output can be stabilized at a certain magnitude by detecting the laser output and controlling the charging voltage of the capacitor.
However, when the laser is operated for a long period of time, the oscillation efficiency is reduced along with the reduction of the halogen gas as mentioned above, which results in that the laser output cannot be maintained at a desired level so long as the charging voltage is not correspondingly increased gradually. Therefore, when the charging voltage is increased to such a predetermined threshold value that a reduction in the oscillation efficiency cannot be compensated for only by an increase in the charging voltage, a predetermined amount of halogen gas is supplied into the laser chamber.
A method of replenishing the halogen gas in the prior art technique will be explained by referring to FIGS. 17 to 19.
FIG. 17 shows an arrangement of a general fluorine-series excimer laser system associated with its gas supply. In this case, there are provided, as gas injection cylinders, a cylinder 8 filled with a halogen gas (such as F2) diluted with a buffer gas (such as Ne), a cylinder 9 filled with such a rare gas as Kr, and a cylinder 10 filled with such a buffer gas as Ne. When it is desired to inject gases prior to starting of the laser, on-off valves 11, 12 and 13 are controllably opened and closed to supply the gases to a laser chamber 4; while, when it is desired to replenish the laser chamber 4 with the halogen gas after operation of the laser, on-off valves 17 and 16 are controllably opened and closed to supply the halogen gas to the laser chamber 4 via a sub-tank 15.
When it is desired to newly inject the gases into the laser chamber 4 prior to the starting of the laser, first of all, old gases present within the laser chamber 4 are discharged by a vacuum pump 20 from the chamber 4 via an on-off valve 19. Subsequently, 40 Torr of Kr gas is charged from the gas cylinder 9 through the on-off valve 12 into the laser chamber 4, 80 Torr of F2 gas diluted with an Ne gas is charged from the gas cylinder 8 through the on-off valve 11 into the chamber 4, and then an Ne gas is charged from the gas cylinder 10 through the on-off valve 13 into the laser chamber 4 so that a total of gas pressure within the chamber 4 becomes 2500 Torr.
As a result of such gas injection control, the laser chamber 4 of the laser system has a partial pressure ratio F2:Kr:Ne of 4:40:2456 (Torr) or 0.16:1.60:98.24 (%).
After the laser chamber 4 is filled with such new gases, the operation of the laser system will be carried out in accordance with a flowchart of FIG. 18 showing how to control gas replenishment.
Prior to the operation of the excimer laser system, first, a target laser output Ec, an optimum control charging voltage range Vc (Vmin to Vmax), an increment/decrement charging voltage .DELTA.V by one control operation, and a replenishment gas amount .DELTA.G by one control operation are previously set (step 901).
When the laser system then starts its operation, a laser output E detected by a laser output monitor 6 as well as a charging voltage V detected by a charging voltage detector 24 are supplied to a controller 1 (step 902). The controller 1 compares the detected laser output E with the target laser output Ec (step 903), adds the aforementioned fine voltage .DELTA.V to the detected charging voltage V so that the voltage V becomes a command charging voltage Va when E&lt;Ec (step 904), uses the detected charging voltage V as the command charging voltage Va as it is when E=Ec (step 905), and subtracts the aforementioned fine voltage .DELTA.V from the detected charging voltage V so that the voltage V becomes the command charging voltage Va when E&gt;Ec (step 906).
Further, the controller 1 compares the command charging voltage Va with a maximum value Vmax within the optimum control charging voltage range Vc (step 907), and when Va.ltoreq.Vmax, returns to the step 902 to control the command voltage Va. When Va&gt;Vmax, however, the controller 1 controls to cause the Ne gas containing the F.sub.2 gas to be replenished by the predetermined amount .DELTA.G from the gas cylinder 8 to the laser chamber 4 (step 908) and at the same time, to cause the gas within the laser chamber 4 to be partly discharged so that the total gas pressure in the laser chamber 4 is kept at a predetermined level (step 909).
FIG. 19 is a timing chart for explaining the timing of the laser output E, command charging voltage Va, halogen gas (F2) partial pressure, rare gas (Kr) partial pressure under the above control. In the drawing, a time axis is represented by the number of shots of the laser. In part (b) of FIG. 19, time points t1 to t6 at which the charging voltage command Va to the capacitor abruptly drops, correspond to the gas replenishing timing of the halogen gas respectively.
In the aforementioned prior art, however, the gas cylinder 8 for replenishment of the halogen gas contains no rare gas component (Kr gas in the above example). In addition, the prior art controls to keep the total internal pressure of the laser chamber 4 constant by partly discharging the gas in the chamber 4 for each gas replenishment (step 909 in FIG. 18), which results in that, as the gas replenishment is frequently carried out, the rare gas (Kr) is gradually decreased as shown in part (d) of FIG. 19, and mere rough calculation shows that 10 times of gas replenishment leads to loss of 20% of Kr gas. In other words, this means that, as the gas replenishment is carried out, the halogen gas (F2) is gradually excessively supplied as shown in part (c) of FIG. 19. Further, even when one time of replenishment .DELTA.G is set to be small so as to prevent excessive replenishment, the replenishment interval becomes gradually shorter as shown in part (c) of FIG. 19, which eventually leads to the change in the composition balance of gases. That is, in the prior art, each time the halogen gas is replenished, the optimum composition balance of the mixture gas in the laser chamber 4 is destroyed, which results in that, even the charging voltage to the capacitor is controlled, the laser output cannot be kept constant.
Furthermore, in the prior art, the gas replenishing time point is determined by the charging voltage comparison (step 907) independently of the optimum gas composition balance and the comparing time point of the charging voltage is based on the laser output comparison (step 903) also independently of the optimum gas composition balance. In particular, the laser output comparison, which is influenced by impurity gases generated within the laser chamber 4, the stain of optical elements thereof, etc., can have great effect on the charging voltage comparison. That is, the prior art gas replenishing method has had such a problem that such an indirect physical quantity as mentioned above is unnaturally used as a reference of deciding the gas replenishing time point, which unfavorably results in that the gas balance is destroyed by the above disturbances.
In the case of a demagnification projection aligner, there has been a problem that the spectrum width, which has a great influence on an imaging performance of a projection alignment lens, also changes along with the fluctuation of the laser gas composition.
Further, in the case of a narrow-band excimer laser used in the demagnification projection aligner, there has been a problem that, when the gas condition changes, the spectrum width becomes narrower as the number of operating shots increases. As shown in FIG. 6, each time the gas is replenished the spectrum width .DELTA..lambda. instantaneously becomes large and thus a stable spectrum width cannot be obtained.
On the other hand, in the case of an excimer laser system, the halogen decrease rate in the initial operating stage is usually unstable. This is because the temperatures of constituent parts of the laser including a laser chamber do not reach a stable level yet and the reaction rates of the halogen gas with the respective parts within the laser system are dependent on the temperatures. For this reason, the decrease rate of the halogen gas here is not a constant value.